Toshiba Bombeat coming back to life

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caution

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Mar 25, 2014
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I picked up this fella over two years ago and thought I'd finally see if I can figure out what's going on. It's an extremely rare red version of the RT-S653/Bombeat DM-02. The only other photo I've been able to find is on Jen's boombox database. The thing uses the same volume pot as the M90, and also like the M90 and others it uses conductive ink for wiring and printed carbon for resistors on the top side of the board.

It's in a coma. When you turn it on, you get a click from the switch bounce and the quietest hiss you can possibly detect with your ear right up to the speaker, no matter what the volume is set to. After checking some voltage levels on the amp chip, it actually checked out okay. It seemed to be at least making super-quiet hiss and not silence, so I checked all the diodes. All except for two of them came up with acceptable voltage drops, both 3.3V zeners. I yanked out one side of them to get a solid reading. Yep, they're really dead. New 3.3V zeners were installed, but still in a coma.

I took a closer look at the area of the circuit where the dead diodes were. They're connected to an NPN transistor, Q901, that ties directly to the DC input jack. The datasheet says it's an amplifier. Hmm. Well, I'm lacking amplification, so maybe someone plugged in the wrong polarity into the DC jack and fried the diodes AND Q901? The only other way to power it is C cells, so I figured that may be what happened. The power LED doesn't even come on and it's right on the other side of Q901. Fishy stuff.

So I pulled out Q901, verified the pin order (ECB) and stuck it in the transistor checker on my multimeter and got no gain at all. Zee-ro. I tested a brand new spare NPN transistor of another type and got a gain within the specced range on its datasheet, so I know the meter works. No match in my pile of spares, so ebay to the rescue. They showed up today, but the package felt a lot heavier than it should have for their tiny size. Sure enough, the seller sent me 2SC2665 instead of 2SC2655, which are about five times larger than the correct ones. Woops. Honest mistake by an honest seller. The right ones ship tomorrow.

The belts were also a licorice factory inside, so I measured their size with some wire and found some good replacements from a VCR I tore apart recently, and if you've ever seen a VCR belt, they're BEEFY! All the pulleys were large enough to accept them, no clearance issues, so done and done.

When I popped the knobs and caps off, I unknowingly destroyed the locking mechanism inside the two pushbutton switches for the stereo modes. As it turns out, if you leave them in the engaged position, pulling on the shaft is very, very bad.

I went through my spare switches and found one match, although it had shorter leads. Not a problem, as it turns out it's the easiest switch I've ever disassembled, you only have to disengage a single tab in the back and it all comes apart. All I need is the shaft anyway.

In the top of the switch case there is a spring-loaded brass pin that swivels left and right (up and down is fixed) and is centered over the track. I took a couple of pictures at different angles so the slopes in the groove floor can be seen. When the switch is in the disengaged position, the pin is sitting at the south end in a pit.

When you press in the button to engage it, the pin moves up the right side of the track, which slopes up. it falls over the cliff in the northeast corner, and then as you let go, it falls over another cliff and settles into the pit at the top center. It's at this engaged position that any pulling on the shaft will make the pin snap the center plastic piece clean off, as shown. It happened when I pushed a screwdriver into their caps from behind to pop them off.

Finally, pushing the button again to disengage it moves the pin over the cliff and into the pit in the northwest corner, so when you let go, the pin goes down the left side of the track and over the last cliff in the southwest corner, settling back into the south end.

Unfortunately I broke two of them, and I could only find one extra in my pile of spare switches. It's nearly impossible to find replacement switches based on the switch manufacturer, in this case it's SMK, and this doesn't even seem to have a part number on it, possibly because it was a custom version made specifically for Toshiba. They have a part number for it, but no dice there.

Then, I remembered that the plastic cap was still on the spare I found, and it said "POWER" so I tried to figure out which tape deck I tore that out of. I zoomed in on the pic of it I took before tearing it down and found it, a Sony TC-FX170. A cross-reference turned up four more Sony tape decks that used it, so off to ebay. I found some decks I could have paid upwards of $30 all told to get, but then I found that someone had torn down one of them and was selling just the power switch board for a few bucks. Yes please!

I was hoping to take care of the transistor this weekend as I waited for the new switch to show up, but due to seller error things will be on hold for a bit. More to come!












 

caution

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Mar 25, 2014
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Pushbutton switches are all fixed and reinstalled, as well as a new transistor, When I was soldering in the new one today I noticed the printed resistor next to it was totally destroyed and an open circuit. I found the correct value on the schematic and installed a replacement into an adjacent set of holes that are in parallel with it, for a cap which they decided to solder on the back.

Fired it up and for one brief moment I heard an FM station start quiet and fade away altogether in about one second, so it's starting to wake up. I guess I shouldn't be surprised there are still bad parts afoot, so will have to do some more sleuthing. I have some replacement main amps and muting transistors just in case, but the way it's not getting adequate power to anything makes me think the power input area still needs some attention.


 

caution

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Mar 25, 2014
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Two out of the three pins on the transistor I replaced were reading in the millivolts when they should have been 6-7V. Something was holding one of these to ground, but there are a lot of parts tied between these two nodes, so I just started with the big cap right there. Looked fine, but once I pulled it, there was a small amount of dried up crustiness. Shorted cap. I swore you could run electrolytics backwards for some time and they'll be okay, but who knows how long this had -9V going in instead of +9V.

Threw a Nichicon goldie in there. So that makes two diodes, a resistor, a transistor, and an electrolytic. And two broken switches of my own doing.

The glowing LEDs made my night!




I'll give her a good check tomorrow to see if any other problems remain, but the radio tuned nice and strong on headphones.
 

caution

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After getting everything back together, the left channel gave out. I pulled the case apart and noticed why the case screws started giving so hard so soon - the shafts for the deck keys had slipped off the buttons on the chassis and was not seating properly inside, causing things to not fit right and put pressure in spots. Since I could still hear the left channel a very small amount, I knew it was working, just struggling. Perhaps the muting transistor failed.

After I got it apart, BOTH channels were next to dead, with one slightly, slightly more audible than the other. If I turned on the unit you couldn't hear anything at all until pressing the record bar in and out once, then faint sounds start coming out. So, my suspicion was that the muting transistors finally died after being marginal from damage they incurred when the zeners tied to them went tits up.

Pulled one of the four out, gave it a test. Dead.

So, tonight I will test the other three and try some new ones. Glad I picked up a few.
 

T-STER

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Jul 14, 2014
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This is a lot of care and attention for a little tosh, i hope you save it after all the work :thumbsup:
 

caution

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Thanks T-STER! Yeah, toasted parts are good at playing hide-and-seek with you, it's like a big box of chocolates :-) And who doesn't like chocolate?
 

T-STER

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For what its worth, threads like this are how i am gradually learning more and more about the actual electronics and conducting more and more thorough repairs. You may be helping yourself to fix a boomer but indirectly your helping me to. I follow these threads with interest.
 

caution

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Mar 25, 2014
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Sweet, good to know I'm helping! Get ready for another lesson in troubleshooting :-)

The power input transistor I replaced is actually fine. The leads weren't straight enough to make full contact in the tester. I got suspicious when I was testing the muting transistors. All four came up dead, and then they all came up fine, after some straightening with a pliers.

Aux in, radio and tape all work fine, but the output is still dead on the left and very, very quiet on the right channel. The amp's fine, but the signal getting to it from the preamp was too low, because I could hear hiss almost as loud as the audio.

I thought perhaps the PCB bent just enough to develop a crack somewhere on the top layer, with all the silver paint and resistors. Everything I tested was coming up fine, so I ditched that idea. I never felt like I forced anything when reassembling anyway, and never heard any snapping or cracking.

I started to lose the notion that reassembling it was actually the cause of my problem. Maybe the preamp chip might be bad? The inputs seemed dead, but I could hear loud crackles out of the left side just rubbing my finger in the area, so I ruled out the chip itself. Poking around the chip some more, I noticed that if I held a bare metal pick to the negative inputs, the right side went full blast (on a second radio station I hadn't tuned in and didn't change when I turned the dial, weird!) but when I touched the left channel, it killed the RIGHT channel, and I had to power cycle it to get the audio back.

Maybe that was related to having to press the record bar in and out at least once, sometimes twice, after turning it on to get any audio?

I started thinking one or more of the smaller electrolytics on the audio path before the preamp had died, so I started with the left channel, being the completely dead one, and tested their resistance in-circuit. One of them was about 5 ohms, which, where it was in the circuit, didn't seem right. The others were all over 10k.

Sure enough, it was the same resistance out of the circuit, so I popped a new one in and SUCCESS!!!

Both channels came alive at full power. Why that cap decided to finally die at the very moment I reassembled it was bizarre, and why that bad cap was pulling down the right side with it, maybe through the preamp chip, made it hard to troubleshoot.

As a side note, as I was looking over the caps, I noticed that Toshiba accidentally installed a .022 uF ceramic cap where there should have been a 100uF electrolytic. Its function isn't entirely clear, it just goes to ground off a preamp pin tied internally to the preamp's power pin, so it might help provide stable power to it. You can even see the circular symbol marking for an electrolytic.



Decided to blacken the speakers while I was in there...


Gross!


So all in all:

Two broken pushbuttons (of my own doing)
Two shorted electrolytics
Two open zeners
An incorrect cap
A fried printed resistor, replaced with a 1/4W metal film
 

T-STER

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Brilliant, so shes all up and running? This was a cracking job, heartwarming to see another one saved :thumbsup:
 

caution

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Mar 25, 2014
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Yep all back together and on the shelf rockin' away now. But, the deck is a tiny bit fast, so I'll have to tweak that.

Even though it's a tone box, the loudness circuit makes up for it. The speakers shake hard and the tweeters are so strong on stereo wide that you have to take it into the bass side a bit.
 
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